Solid-state image pickup device and camera system

ABSTRACT

A solid-state image pickup device including a pixel unit in which a plurality of photoelectric conversion elements having different sensitivities are arranged; and a pixel reading unit configured to read and add output signals from the plurality of photoelectric conversion elements in the pixel unit, and to obtain an output signal seemingly from one pixel. The pixel unit includes an absorbing unit configured to absorb overflowing electric charge from a photoelectric conversion element with a high sensitivity.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.12/698,326 filed Feb. 2, 2010, the entirety of which is incorporatedherein by reference to the extent permitted by law. The presentapplication claims the benefit of priority to Japanese PatentApplication No. JP 2009-027895 filed on Feb. 9, 2009 in the Japan PatentOffice, the entirety of which is incorporated by reference herein to theextent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pickup devicerepresented by a complementary metal-oxide semiconductor (CMOS) imagesensor, and to a camera system.

2. Description of the Related Art

In recent years, CMOS image sensors have been attracting attention assolid-state image pickup devices (images sensors), in place ofcharge-coupled devices (CCDs).

This is because CMOS image sensors overcome the following problems.

That is, the problems include that a dedicated manufacturing process isnecessary for fabricating CCD pixels, a plurality of power supplyvoltages are necessary for the operation thereof, and it is necessary tocause a plurality of peripheral integrated circuits (ICs) to be operatedin a combined manner.

CMOS image sensors overcome these various problems of CCDs, such as thatthe system becomes very complicated.

CMOS image sensors can be manufactured using a manufacturing processsimilar to the process of manufacturing general CMOS ICs. Also, a CMOSimage sensor can be driven by a single power supply. Furthermore, ananalog circuit and a logic circuit using CMOS processes can be mixed ina single chip.

Accordingly, the number of peripheral ICs in a CMOS image sensor can bereduced. That is, CMOS sensors have multiple great advantages.

An output circuit of a CCD is generally a 1-channel (ch) output using afloating diffusion (FD) amplifier with a floating diffusion layer.

In contrast, a CMOS image sensor has an FD amplifier in each pixel andgenerally uses a column-parallel output scheme that selects a row froman array of pixels and simultaneously reads and outputs signals from therow in a column direction.

Because it is difficult to obtain sufficient drive power using the FDamplifiers arranged in the pixels, the data rate is necessary to bedropped. In this sense, parallel processing is regarded to beadvantageous.

Such CMOS image sensors have been widely used as image pickup devices inimage capturing apparatuses such as digital cameras, camcorders,monitoring cameras, and in-vehicle cameras.

The technique of adding output signals from multiple photodiodes (PDs)with different sensitivities and outputting the sum signal as an outputsignal from a pixel is effective as a method of realizing a CMOS imagesensor with a high dynamic range. In particular, buried photodiodes(BPDs) are widely used as PDs. Since there is a surface level due todefects such as dangling bonds on the surface of a substrate on whichPDs are formed, a great amount of electric charge (dark current) isgenerated owing to the thermal energy. As a result, it becomes difficultto read a correct signal. In the case of BPDs, electric chargeaccumulating portions of PDs are buried in the substrate. In this way,the amount of dark current introduced into the signal is reduced.

The sensitivity of a PD can be changed by changing the exposure time orby providing a neutral density (ND) filter.

This method has the following advantages:

A higher dynamic range than that achieved by simply using a large pixelcan be achieved; and

Although the output relative to the amount of incident light isnonlinear, the output can be easily changed back to be linear. When acolor image is obtained, it is easy to perform color processing.

SUMMARY OF THE INVENTION

When there is overflowing electric charge from a BPD with a highsensitivity, the overflowing electric charge flows into a BPD with a lowsensitivity. It thus becomes difficult to output correct data.

In contrast, when the exposure time is reduced so that no overflowingelectric charge will be generated and a BPD with a high sensitivity willnot be saturated, the dynamic range is not extended.

The present invention provides a solid-state image pickup device and acamera system that can absorb overflowing electric charge from aphotoelectric conversion element with a high sensitivity, that canrealize a correct data output, and that can realize a high dynamicrange.

A solid-state image pickup device according to an embodiment of thepresent invention includes a pixel unit in which a plurality ofphotoelectric conversion elements having different sensitivities arearranged; and a pixel reading unit configured to read and add outputsignals from the plurality of photoelectric conversion elements in thepixel unit, and to obtain an output signal seemingly from one pixel. Thepixel unit includes an absorbing unit configured to absorb overflowingelectric charge from a photoelectric conversion element with a highsensitivity.

A camera system according to an embodiment of the present inventionincludes a solid-state image pickup device; an optical system configuredto form an image of a photographic subject on the solid-state imagepickup device; and a signal processing circuit configured to process anoutput image signal of the solid-state image pickup device. Thesolid-state image pickup device includes a pixel unit in which aplurality of photoelectric conversion elements having differentsensitivities are arranged, and a pixel reading unit configured to readand add output signals from the plurality of photoelectric conversionelements in the pixel unit, and to obtain an output signal seeminglyfrom one pixel. The pixel unit includes an absorbing unit configured toabsorb overflowing electric charge from a photoelectric conversionelement with a high sensitivity.

According to an embodiment of the present invention, overflowingelectric charge from a photoelectric conversion element with a highsensitivity can be absolved; a correct data output can be realized; anda high dynamic range can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a structure example of a CMOS imagesensor (solid-state image pickup device) according to an embodiment ofthe present invention;

FIG. 2 is a diagram illustrating an example of a pixel circuit of theCMOS image sensor according to the present embodiment;

FIG. 3 is a diagram illustrating an arrangement example of the pixelcircuit according to a first embodiment;

FIG. 4 is a graph illustrating an example of an output from each pixel;

FIG. 5 is a diagram illustrating the generation of overflowing electriccharge;

FIGS. 6A and 6B are diagrams describing an overflow path of the firstembodiment;

FIG. 7 is a diagram describing the overflow path of the first embodimentand illustrating the potential of electrons along line VIB-VIBillustrated in FIG. 6A;

FIG. 8 includes diagrams describing the overflow path of the firstembodiment and illustrating the potential along line VIII-VIIIillustrated in FIG. 6B;

FIG. 9 includes timing charts, according to the first embodiment,illustrating an example of the case where the sensitivity of each BPD ischanged in accordance with an exposure time;

FIG. 10 includes timing charts, according to the first embodiment,illustrating an example of the case where the sensitivity of each BPD ischanged by providing a neutral density (ND) filter or the like;

FIGS. 11A and 11B are diagrams describing an overflow path of a secondembodiment;

FIG. 12 is a diagram describing the overflow path of the secondembodiment and illustrating the potential of electrons along lineXIB-XIB illustrated in FIG. 11A;

FIG. 13 includes diagrams describing the overflow path of the secondembodiment and illustrating the potential along line XIII-XIIIillustrated in FIG. 11B;

FIGS. 14A and 14B are diagrams describing an overflow path of a thirdembodiment;

FIG. 15 includes diagrams describing the overflow path of the thirdembodiment and illustrating the potential of electrons along lineXIVB-XIVB illustrated in FIG. 14A;

FIG. 16 includes diagrams describing the overflow path of the thirdembodiment and illustrating the potential along line XVI-XVI illustratedin FIG. 14B;

FIG. 17 includes timing charts, according to the third embodiment,illustrating an example of the case where the sensitivity of each BPD ischanged in accordance with an exposure time;

FIG. 18 includes timing charts, according to a fourth embodiment,illustrating an example of the case where the sensitivity of each BPD ischanged by providing an ND filter or the like;

FIG. 19 is a diagram illustrating an arrangement example of a pixelcircuit according to the fourth embodiment in the case where four BPDswith different sensitivities are shared by one floating diffusion (FD);

FIG. 20 includes exemplary timing charts according to the fourthembodiment;

FIGS. 21A and 21B are diagrams describing an overflow path of a fifthembodiment;

FIG. 22 includes diagrams describing the overflow path of the fifthembodiment and illustrating the potential of electrons along lineXXIB-XXIB illustrated in FIG. 21A;

FIG. 23 is a diagram illustrating an arrangement example of a pixelcircuit according to a sixth embodiment;

FIGS. 24A and 24B are diagrams describing an overflow path of the sixthembodiment;

FIG. 25 is a diagram describing the overflow path of the sixthembodiment and illustrating the potential of electrons along lineXXIVB-XXIVB illustrated in FIG. 24A;

FIG. 26 is a diagram illustrating an example of a pixel circuit of aCMOS image sensor according to a seventh embodiment;

FIG. 27 is a diagram illustrating an arrangement example of the pixelcircuit according to the seventh embodiment;

FIGS. 28A and 28B are diagrams describing an overflow path of theseventh embodiment;

FIG. 29 includes diagrams describing the overflow path of the seventhembodiment and illustrating the potential of electrons along lineXXVIIIB-XXVIIIB illustrated in FIG. 28A;

FIG. 30 is a block diagram illustrating a structure example of asolid-state image pickup device (CMOS image sensor) includingcolumn-parallel analog-to-digital converters (ADCs) according to aneighth embodiment; and

FIG. 31 is a diagram illustrating an example of the configuration of acamera system to which the solid-state image pickup device according toan embodiment of the present invention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe drawings.

The description will be given in the following order:

1. First Embodiment

2. Second Embodiment

3. Third Embodiment

4. Fourth Embodiment

5. Fifth Embodiment

6. Sixth Embodiment

7. Seventh Embodiment

8. Eighth Embodiment

9. Ninth Embodiment

1. First Embodiment

FIG. 1 is a diagram illustrating a structure example of a CMOS imagesensor (solid-state image pickup device) according to an embodiment ofthe present invention.

A CMOS image sensor 100 includes a pixel array section 110, a rowselecting circuit (Vdec) 120 serving as a pixel driving unit, and acolumn reading circuit (AFE) 130.

The pixel array section 110 includes multiple pixel circuits 110A whichare arranged in two dimensions (matrix) of M rows×N columns.

A resetting control line LRST, a transfer control line LTRG, and aselection control line LSEL arranged in the pixel array section 110 aregrouped as a set for each row of the pixel arrangement.

The resetting control line LRST, the transfer control line LTRG, and theselection control line LSEL are driven by the row selecting circuit 120.

The row selecting circuit 120 controls the operation of pixels arrangedon an arbitrary row in the pixel array section 110. The row selectingcircuit 120 controls the pixels through the control lines LSEL, LRST,and LTRG.

FIG. 2 is a diagram illustrating an example of a pixel circuit of theCMOS image sensor according to the present embodiment.

The pixel circuit 110A includes four BPDs 111 a to 111 d for performingphotoelectric conversion.

The pixel circuit 110A includes transfer transistors (TGs) 112 a to 112d which are individually provided for the BPDs 111 a to 111 d. The pixelcircuit 110A also includes a reset transistor 113, an amplifyingtransistor 114, and a selection transistor 115 as active elements.

The pixel circuit 110A is formed as a sharing pixel circuit in which thefour BPDs 111 a to 111 d share the reset transistor 113, the amplifyingtransistor 114, and the selection transistor 115.

When the pixel circuits 110A are arranged in two dimensions of M rows×Ncolumns, M control lines LRST, M control lines LSEL, and 4M controllines LTR are provided.

The BPDs 111 a to 111 d perform photoelectric conversion of convertingincident light into electric charge (electrons in this case) whoseamount is in accordance with the amount of the incident light.

The BPDs 111 a to 111 d are connected to a floating diffusion FD via thetransfer transistors 112 a to 112 d, respectively.

The transfer control lines LTRGa to LTRGd are connected to the gates ofthe transfer transistors 112 a to 112 d, respectively.

The transfer transistors 112 a to 112 d transfer electrons obtained byphotoelectric conversion performed by the BPDs 111 a to 111 d inaccordance with the potentials of the transfer control lines LTRGa toLTRGd to the floating diffusion FD.

The reset transistor 113 is connected between a power supply line LVDDand the floating diffusion FD.

The reset transistor 113 resets the potential of the floating diffusionFD in accordance with a potential applied to the reset control line LRSTto the potential VDD of the power supply line LVDD.

The gate of the amplifying transistor 114 is connected to the floatingdiffusion FD.

The amplifying transistor 114 is connected via the selection transistor115 to a signal line LVSL.

When the selection transistor 115 is turned on in accordance with theselection control line LSEL, the amplifying transistor 114 outputs asignal in accordance with the potential of the floating diffusion FD tothe signal line LVSL.

A voltage output from each pixel is output through the signal line LVSLto the column reading circuit 130.

The column reading circuit 130 converts an analog signal output to thesignal line LVSL into a digital signal and outputs the digital signal.

Hereinafter, the case where electric charge accumulated in a BPDincludes electrons will be described. However, an embodiment of thepresent invention is also applicable to the case where the electriccharge includes holes. In that case, it is only necessary to switch aP-type semiconductor and an N-type semiconductor.

FIG. 3 is a diagram illustrating an arrangement example of the pixelcircuit according to the present embodiment.

In the example in FIG. 3, the BPDs 111 a to 111 d are arranged in asquare of 2×2 in each pixel. The floating diffusion FD is arranged inthe center of the four BPDs 111 a to 111 d.

The column reading circuit 130 includes an analog-to-digital converter(ADC) provided in each column.

The BPDs 111 a to 111 d have different sensitivities a to d. Thesensitivities of the BPDs 111 a to 111 d can be changed, for example, byproviding ND filters and changing the amount of incident light, or bychanging the exposure time.

Signals detected by the BPDs 111 a to 111 d are added by an ADC in eachcolumn, and the sum signal is output.

FIG. 4 is a graph illustrating an example of an output from each pixel.

In FIG. 4, the amount of incident light is plotted in abscissa, and anoutput signal is plotted in ordinate.

FIG. 4 illustrates the case where the resolution of an ADC at the timeof reading signals from the BPDs 111 a to 111 d is 10 bits, and thesensitivity ratio a:b:c:d of the BPDs 111 a to 111 d is a:b:c:d=8:4:2:1.

The dynamic range of the sensor is determined by the maximum value andthe minimum value of the amount of light that can be read.

According to the structure of the first embodiment, while the minimumvalue of the amount of light that can be read remains substantiallyunchanged, the maximum value is increased by eight times. Accordingly,the dynamic range can be extended.

However, the method of adding signals detected by the BPDs 111 a to 111d with different sensitivities has the problem that a BPD with a highsensitivity becomes saturated as the amount of light increases, leadingto the generation of overflowing electric charge.

For example, in FIG. 4, when the amount of light is within the range of1× to 2×, as illustrated in FIG. 5, overflowing electric charge isgenerated in the BPD 111 a.

Unless the overflowing electric charge is absorbed, the overflowingelectric charge flows into peripheral pixels. It thus becomes difficultto obtain a correct output value.

In contrast, in the first embodiment, an overflow path OFP is providedas an absorbing unit from each of the BPDs 111 to the floating diffusionFD, and overflowing electric charge generated in each of the BPDs 111 isdischarged to the floating diffusion FD.

An absorbing unit is formed by using an overflow path that absorbsoverflowing electric charge in each of the BPDs 111.

FIGS. 6A and 6B are diagrams describing the overflow path of the firstembodiment. FIG. 6A is a top view of a pixel according to the firstembodiment, and FIG. 6B is a sectional view of the BPD 111, the transfertransistor (TG) 112, and the floating diffusion FD taken along lineVIB-VIB illustrated in FIG. 6A.

Also, FIG. 7 is a diagram describing the overflow path of the firstembodiment and illustrating the potential of electrons along lineVIB-VIB illustrated in FIG. 6A.

FIG. 8 includes diagrams describing the overflow path of the firstembodiment and illustrating the potential along line VIII-VIIIillustrated in FIG. 6B.

FIG. 8 illustrates the potential below a transfer gate of the transfertransistor (TG) 112.

In the first embodiment, overflowing electric charge generated in theBPD 111 is discharged through the overflow path provided in the transfertransistor (TG) 112 to the floating diffusion FD.

A positive potential (e.g., power supply voltage) is supplied to thefloating diffusion FD. The overflowing electric charge is dischargedfrom the floating diffusion FD.

By providing the overflow path in the transfer transistor (TG) 112, theoverflowing electric charge can be discharged without increasing thearea.

When the gate poly-Si of the gate (transfer gate) of the transfertransistor (TG) 112 is doped with N-type, it is desirable to apply anegative potential (e.g., −1 V) to the transfer control line LTRG in anoff state, or to dope the gate poly-Si of the transfer gate with P-typeand to apply 0 V.

Since there is a surface level due to a defect such as a dangling bondat the transistor interface, a large amount of electric charge isgenerated by the thermal energy.

Thus, if there is an overflow path at the transistor interface, electriccharge generated from the surface level flows into the BPD 111, and itbecomes difficult to read correct data.

In contrast, when a negative voltage is applied to the transfer gate (orwhen the gate poly-Si is made P-type), as illustrated in FIG. 8, thepotential at the transistor interface of the transfer gate becomeshigher, and holes are accumulated.

Accordingly, the generation of electric charge at the transistorinterface can be suppressed.

However, if the potential at the transistor interface is increased, itbecomes difficult to provide an overflow path at the transistorinterface.

Therefore, the overflow path of the first embodiment is provided at aposition deeper than the transistor interface (Si—SiO₂ interface) of thetransfer transistor (TG) 112, as illustrated in FIG. 6(B) and FIG. 8.

For example, when the depth of the BPD 111 is 2 to 4 μm and the depth ofthe floating diffusion FD is about 0.4 μm, the overflow path OFP isprovided at the depth of about 0.2 to 0.5 μm.

In this way, introduction of noise due to the surface level can beprevented.

Also, since the overflow path OFP is sufficiently distant from a channel(e.g., 200 to 300 nm), there is no effect on the transfer of electriccharge. The overflow path OFP can be formed by injecting a very smallamount of impurity that makes silicon an N-type semiconductor, such asAs.

FIG. 7 is a diagram illustrating the potential of electrons in theVIB-VIB section in the horizontal direction and the depth direction.

The overflow path of the first embodiment is formed such that thepotential of electrons locally becomes lower, compared with peripheralsections.

In this way, if the electric charge accumulated in the BPD 111 exceeds acertain amount, the exceeding amount is discharged through the overflowpath to the floating diffusion FD.

Parts (A) to (F) of FIGS. 9 and 10 are timing charts according to thefirst embodiment.

Parts (A) to (F) of FIG. 9 illustrate an example of the case where thesensitivities a to d of the BPDs 111 a to 111 d are changed inaccordance with the exposure time.

The sensitivity ratio among the BPDs 111 a to 111 d is determined inaccordance with the exposure time: a:b:c:d=Ta:Tb:Tc:Td.

In contrast, parts (A) to (F) of FIG. 10 illustrate an example of thecase where the sensitivities a to d of the BPDs 111 a to 111 d arechanged by providing, for example, ND filters.

In this case, the exposure times of the BPDs 111 a to 111 d are madeequal as T.

During an exposure period, the resetting control line LRST is caused tobe at a high level (H), thereby turning on the reset transistor 113.Accordingly, the power supply potential VDD is supplied to the floatingdiffusion FD.

When reading electric charge from the BPDs 111, it is necessary to turnoff the reset transistor 113 and to separate the floating diffusion FDfrom the power supply line LVDD.

Thus, if reading is not performed in an appropriate order, overflowingelectric charge OFC discharged through the overflow path OFP to thefloating diffusion FD is introduced into the electric charge transferredfrom the BPDs 111 to the floating diffusion FD. Therefore, in the firstembodiment, reading from the BPDs 111 is performed in the descendingorder of sensitivity.

For example, when the levels of the sensitivities satisfy therelationship a>b>c>d, signals are read from the BPDs 111 a to 111 d inthe descending order of sensitivity: BPD 111 a, BPD 111 b, BPD 111 c,and BPD 111 d.

In this way, even when the overflowing electric charge OFC is introducedinto the electric charge transferred from the BPDs 111, a correct outputvalue can be obtained from the ADC.

For example, when the output value relative to the amount of light hasthe characteristics illustrated in FIG. 4, under the condition that theamount of light 2× to 4× is incident, overflowing electric charge OFCmay be generated in the BPD 111 a and the BPD 111 b.

However, the BPD 111 c and the BPD 111 d are not saturated, and nooverflowing electric charge is generated.

Under this condition, when electric charge is first read from the BPD111 a, the overflowing electric charge OFC from the BPD 111 b isintroduced into the floating diffusion FD. However, since the output ofthe BPD 111 a is saturated, the value read from the ADC remains 1024,which is unchanged.

Next, when a signal is read from the BPD 111 b, the electric charge hasalready been read from the BPD 111 a, and the BPD 111 a is notsaturated. Thus, introduction of the overflowing electric charge OFCdoes not occur.

Similarly, when electric charge is read from the BPD 111 c and from theBPD 111 d, since there is no BPD 111 that is saturated, introduction ofthe overflowing electric chare OFC does not occur.

Thus, the output value from the ADC is not affected by the overflowingelectric charge, and a correct output value can be obtained.

As described above, according to the first embodiment, the followingadvantages can be achieved in the CMOS image sensor whose dynamic rangeis extended by adding outputs from multiple BPDs with differentsensitivities.

According to the first embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD to thepower supply.

By using the transfer transistors 112 and the floating diffusion FD asthe overflow path OFP, the overflowing electric charge can beappropriately processed without increasing the area.

By separating the overflow path OFP from the transistor interface,introduction of noise due to the surface level can be prevented.

By reading signals from the BPDs in descending order of sensitivity, theoverflowing electric charge is prevented from being introduced into thefloating diffusion FD, and a correct output value can be obtained.

Although the case in which BPDs are used as elements for performingphotoelectric conversion has been described above, the first embodimentis also effective in the case where PDs that are not buried are used.

The case in which signals from the BPDs are read by using the ADC andare added has been described. Alternatively, the method of processingthe overflowing electric charge OFC by using the transfer transistorsand the floating diffusion FD is also effective in the case wheresignals from the BPDs are simultaneously read to the floating diffusionFD and are added.

2. Second Embodiment

Next, a second embodiment of the present invention will be described.

The overall structure of a CMOS image sensor according to the secondembodiment can be the structure illustrated in FIG. 1, as in the firstembodiment.

The structure of a pixel circuit according to the second embodiment canbe the structure illustrated in FIG. 2, as in the first embodiment.

The arrangement of the pixel circuit according to the second embodimentcan be the arrangement illustrated in FIG. 3, as in the firstembodiment.

The sensitivities a to d of the BPDs 111 a to 111 d according to thesecond embodiment are different, as in the first embodiment.

The output signal and the dynamic range of a pixel according to thesecond embodiment are the same as the first embodiment, as illustratedin FIG. 4.

Also in the second embodiment, overflowing electric charge is generatedfrom the BPD 111 with a high sensitivity, as illustrated in FIG. 5.

FIGS. 11A and 11B are diagrams describing an overflow path of the secondembodiment.

FIG. 11A is a top view of a pixel according to the second embodiment,and FIG. 11B is a sectional view of the BPD 111, the transfer transistor(TG) 112, and the floating diffusion FD taken along line XIB-XIBillustrated in FIG. 11A.

Also, FIG. 12 is a diagram describing the overflow path of the secondembodiment and illustrating the potential of electrons along lineXIB-XIB illustrated in FIG. 11A.

FIG. 13 includes diagrams describing the overflow path of the secondembodiment and illustrating the potential along line XIII-XIIIillustrated in FIG. 11B.

FIG. 13 illustrates the potential below a transfer gate of the transfertransistor (TG) 112.

Also in the second embodiment, overflowing electric charge OFC generatedin the BPD 111 is discharged through the overflow path OFP formed at thetransfer gate which is the transfer transistor 112 to the floatingdiffusion FD.

A positive potential (e.g., power supply voltage) is supplied to thefloating diffusion FD. The overflowing electric charge is dischargedfrom the floating diffusion FD.

By providing the overflow path in the transfer transistor 112, theoverflowing electric charge can be discharged without increasing thearea.

Also in the second embodiment, as in the first embodiment, when the gatepoly-Si of the transfer gate is doped with N-type, it is desirable toapply a negative potential (e.g., −1 V) to the transfer control lineLTRG in an off state, or to dope the gate poly-Si of the transfer gatewith P-type and to apply 0 V.

If the potential at the transistor interface is increased, it becomesdifficult to provide an overflow path at the transistor interface.

Therefore, the overflow path OFP of the second embodiment is provided ata position a little deeper than the transistor interface (Si—SiO₂interface) of the transfer transistor (TG) 112, as illustrated in FIG.11(B).

For example, when the depth of the BPD 111 is 2 to 4 μm and the depth ofthe floating diffusion FD is about 0.4 μm, the overflow path OFP isprovided at the depth of about 50 to 100 nm.

The depth may change according to process. Basically, a position alittle deeper than the PD junction on the BPD surface is desirable.

In this way, introduction of noise due to the surface level can beprevented.

Also, the transfer efficiency at the time of turning on the transfertransistor 112 (transfer gate) and transferring the electric charge canbe improved. The overflow path OFP can be formed by injecting a verysmall amount of impurity that makes silicon an N-type semiconductor,such as As.

FIG. 12 is a diagram illustrating the potential in the XIB-XIB sectionin the horizontal direction and the depth direction.

The overflow path of the second embodiment is formed such that thepotential of electrons locally becomes lower, compared with peripheralsections.

In this way, if the electric charge accumulated in the BPD 111 exceeds acertain amount, the exceeding amount is discharged through the overflowpath OFP to the floating diffusion FD.

Timing charts according to the second embodiment are the same as thosein the first embodiment, as illustrated in parts (A) to (F) of FIGS. 9and 10.

As described above, according to the second embodiment, the followingadvantages can be achieved in the CMOS image sensor whose dynamic rangeis extended by adding outputs from multiple BPDs with differentsensitivities.

According to the second embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD to thepower supply.

By using the transfer transistors 112 and the floating diffusion FD asthe overflow path OFP, the overflowing electric charge OFC can beappropriately processed without increasing the area.

By separating the overflow path OFP from the transistor interface,introduction of noise due to the surface level can be prevented.

By reading signals from the BPDs in descending order of sensitivity, theoverflowing electric charge OFC can be prevented from being introducedinto the floating diffusion FD, and a correct output value can beobtained.

Although the case in which BPDs are used as elements for performingphotoelectric conversion has been described above, the second embodimentis also effective in the case where PDs that are not buried are used.

The case in which signals from the BPDs are read by using the ADC andare added has been described.

Alternatively, the method of processing the overflowing electric chargeOFC by using the transfer transistors (TG) 112 and the floatingdiffusion FD is also effective in the case where signals from the BPDsare simultaneously read to the floating diffusion FD and are added.

3. Third Embodiment

Next, a third embodiment of the present invention will be described.

The overall structure of a CMOS image sensor according to the thirdembodiment can be the structure illustrated in FIG. 1, as in the firstembodiment.

The structure of a pixel circuit according to the third embodiment canbe the structure illustrated in FIG. 2, as in the first embodiment.

The arrangement of the pixel circuit according to the third embodimentcan be the arrangement illustrated in FIG. 3, as in the firstembodiment.

The sensitivities a to d of the BPDs 111 a to 111 d according to thethird embodiment are different, as in the first embodiment.

The output signal and the dynamic range of a pixel according to thethird embodiment are the same as the first embodiment, as illustrated inFIG. 4.

FIGS. 14A and 14B are diagrams describing an overflow path of the thirdembodiment.

FIG. 14A is a top view of a pixel according to the third embodiment, andFIG. 14B is a sectional view of the BPD 111, the transfer transistor(TG) 112, and the floating diffusion FD taken along line XIVB-XIVBillustrated in FIG. 14A.

Also, FIG. 15 includes diagrams describing the overflow path of thethird embodiment and illustrating the potential of electrons along lineXIVB-XIVB illustrated in FIG. 14A.

FIG. 16 includes diagrams describing the overflow path of the thirdembodiment and illustrating the potential along line XVI-XVI illustratedin FIG. 14B.

FIG. 16 illustrates the potential below a transfer gate of the transfertransistor (TG) 112.

In the third embodiment, overflowing electric charge OFC is dischargedthrough the transistor interface of the transfer transistor 112, whichserves as the overflow path OFP, to the floating diffusion floatingdiffusion FD, as illustrated in FIGS. 14A to 16.

Specifically, the potential of a channel of the transfer transistor 112is reduced.

In this way, if the electric charge accumulated in the BPD 111 exceeds acertain amount, the exceeding amount is discharged through the channelof the transfer transistor (TG) 112 to the floating diffusion FD.

However, when the transistor interface serves as the overflow path OFP,electric charge generated at the surface level is introduced into theBPD 111.

It is know that the generation of electric charge at the surface levelcan be greatly suppressed by terminating the defect level of thetransistor interface by using hydrogen H or deuterium D.

However, when the terminating process is insufficient or when theterminated H or D drops off, the defect level is left. As a result,noise which has occurred at the surface level is introduced into some ofthe BPDs 111.

Therefore, in the third embodiment, as illustrated in FIG. 14(B) andFIG. 16, the BPD 111 is extended underneath the transfer gate, and theoverflow path OFP is provided in the vertical direction from the BPD 111to the channel.

Furthermore, the potential between the BPD 111 and the transistorinterface is made the highest in the overflow path OFP.

By providing a barrier between the transistor interface and the BPD 111as above, electric charge generated at the transistor interface issuppressed from being introduced into the BPD 111.

The barrier between the BPD 111 and the transistor interface is providednear the transistor interface. When a positive potential is applied tothe transfer control line LTRG, the potential of the barrier greatlychanges. Accordingly, no failure occurs at the time of transfer. Also,regarding the BPD 111 which is not saturated, by applying a negativepotential to the transfer control line LTRG, the generation of electriccharge at the transistor interface can be prevented.

In the third embodiment, as illustrated in FIGS. 14A to 16, the overflowpath OFP is turned on by applying a positive potential or a groundpotential (e.g., 0 V) to the gate of the transfer transistor (TG) 112connected to the saturated BPD 111 with a high sensitivity. Thegeneration of electrons from the surface level is suppressed by applyinga negative potential (e.g., −1 V) to the gate of the transfer transistor112 connected to the BPD 111 with a low sensitivity.

In this way, although noise from the surface level may be introducedinto the BPD 111 with a high sensitivity, noise is hardly introducedinto the BPD 111 with a low sensitivity.

Therefore, whether noise from the surface level is introduced into theBPD 111 with a high sensitivity can be determined by comparing an outputof the BPD 111 with a high sensitivity with an output of the BPD 111with a low sensitivity.

For example, it is assumed that the sensitivity ratio between the BPD111 a and the BPD 111 b is a:b=2:1, and signals read from the BPD 111 aand the BPD 111 b are denoted by Sa and Sb. When incident light thatenters the BPD 111 a is the same as that enters the BPD 111 b, therelationship between Sa and Sb is as follows, taking noise intoconsideration:2(Sb−Sb ^(1/2)−1)<Sa<2(Sb+Sb ^(1/2)+1)  (1)

Therefore, when Sa>2(Sb+Sb^(1/2)+1), it is determined that the electriccharge from the surface level is introduced. Hence, the output value canbe corrected.

Actually, because the amount of incident light that enters each BPD 111is not completely equal, and the amount of light changes because aphotographic subject or the image pickup device itself moves, therelationship between Sa and Sb may be different from expression (1). Itis therefore desirable to provide some margin.

For example, when about 20% margin is provided, if the output value Sabecomes Sa>2.4(Sb+Sb^(1/2)+1) with respect to the output value Sb, theoutput value Sa is corrected.

Parts (A) to (F) of FIG. 17 and parts (A) to (F) of FIG. 18 areexemplary timing charts according to the third embodiment.

Parts (A) to (F) of FIG. 17 illustrate an example of the case where thesensitivities a to d of the BPDs 111 a to 111 d are changed inaccordance with the exposure time.

The sensitivity ratio among the BPDs 111 a to 111 d is determined inaccordance with the exposure time: a:b:c:d=Ta:Tb:Tc:Td.

In contrast, parts (A) to (F) of FIG. 18 illustrate an example of thecase where the sensitivities a to d of the BPDs 111 a to 111 d arechanged by providing, for example, ND filters.

The exposure times of the BPDs 111 a to 111 d are made equal as T.

In FIGS. 17 and 18, the case where the levels of the sensitivitiessatisfy the relationship a>b>c>d is illustrated.

In the BPD 111 a, BPD 111 b, and BPD 111 c, the gate voltages of thetransfer transistors 112 are increased after the resetting so as to turnon the overflow path OFP.

In contrast, in the BPD 111 d, the voltage applied to the gate of thetransfer transistor (TG) 112 is maintained at a low level even after theresetting, and noise from the surface level is prevented from beingintroduced into the BPD 111 d.

In reading periods, voltages applied to the gates of all the transfertransistors 112 a to 112 d (TRGa to TRGd) are maintained at a low level,thereby turning off the overflow path OFP.

In this way, overflowing electric charge is prevented from beingintroduced into the floating diffusion FD in the reading periods.

In exposure periods, the resetting control line LRST is maintained at ahigh level (H), thereby turning on the reset transistor 113.Accordingly, the power supply voltage VDD is supplied to the floatingdiffusion FD.

In the examples of FIGS. 17 and 18, the overflow path OFP of theindividual transfer transistors 112 a to 112 d is turned on even beforeresetting the individual BPDs 111 a to 111 d. Accordingly, theoverflowing electric charge OFC is discharged to the floating diffusionFD.

This prevents the overflowing electric charge OFC from being introducedinto the BPDs 111 a to 111 d when the BPDs 111 a to 111 d are saturatedbefore being reset.

For example, when the BPD 111 b is saturated, unless the overflowingelectric charge generated in the BPD 111 b is absorbed, the overflowingelectric charge is introduced into the BPD 111 a in a period from theresetting of the BPD 111 a to the resetting of the BPD 111 b.

As illustrated in FIGS. 17 and 18, the overflowing electric charge OFCcan be prevented from being introduced into the BPDs 111 a to 111 d byturning on the overflow path OFP in periods prior to resetting theindividual BPDs 111 a to 111 d.

As described above, according to the third embodiment, the followingadvantages can be achieved in the CMOS image sensor whose dynamic rangeis extended by adding outputs from multiple BPDs with differentsensitivities.

According to the third embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD to thepower supply.

By using the transfer transistors 112 and the floating diffusion FD asthe overflow path OFP, the overflowing electric charge OFC can beappropriately processed without increasing the area.

By applying a low voltage to the gate of the transfer transistor 112connected to a BPD with a low sensitivity, noise from the surface levelof the transistor interface can be prevented from being introduced intothe BPD, and a correct output value can be obtained.

Even when noise from the surface level is introduced into a BPD with ahigh sensitivity, a correct output can be obtained by performing acorrection using the output value read from a BPD with a lowsensitivity.

4. Fourth Embodiment

Next, a fourth embodiment of the present invention will be described.

The overall structure of a CMOS image sensor according to the fourthembodiment can be the structure illustrated in FIG. 1, as in the firstto third embodiments.

The structure of a pixel circuit according to the fourth embodiment canbe the structure illustrated in FIG. 2, as in the first to thirdembodiments.

The sensitivities a to d of the BPDs 111 a to 111 d according to thefourth embodiment are different, as in the first embodiment.

The output signal and the dynamic range of a pixel according to thefourth embodiment are the same as the first embodiment, as illustratedin FIG. 4.

Also in the fourth embodiment, overflowing electric charge is generatedfrom the BPD 111 with a high sensitivity, as illustrated in FIG. 5.

An overflow path of the fourth embodiment is the same as that in thethird embodiment. As illustrated in FIGS. 14A to 16, overflowingelectric charge is discharged through the transistor interface of thetransfer transistor 112, which serves as the overflow path, to thefloating diffusion floating diffusion FD.

Also, the point that an output from the BPD 111 with a high sensitivitycan be corrected by using an output of the BPD 111 with a lowsensitivity is the same as the third embodiment.

In a pixel circuit according to the fourth embodiment, the BPD 111 witha high sensitively and the BPD 111 with a low sensitivity are arrangednext to each other.

FIG. 19 is a diagram illustrating an arrangement example of the pixelcircuit according to the fourth embodiment in the case where the fourBPDs 111 a to 111 d with different sensitivities are shared by onefloating diffusion (FD).

FIG. 19 illustrates the case where the levels of the sensitivities ofthe BPDs 111 a to 111 d satisfy the relationship a>b>c>d.

In this case, the BPD 111 a with the highest sensitivity is verticallyand horizontally adjacent only to the BPD 111 c and the BPD 111 d, butnot to the BPD 111 a and the BPD 111 b.

With such a structure, most of the overflowing electric charge generatedin the BPD 111 a or the BPD 111 b flows into the adjacent BPD 111 c orBPD 111 d, and hardly any of the overflowing electric charge flows intothe BPD 111 a and the BPD 111 b.

Parts (A) to (F) of FIG. 20 are exemplary timing charts according to thefourth embodiment.

In the fourth embodiment, the sensitivities a to d of the BPDs 111 a to111 d are changed in accordance with the exposure time. The sensitivityratio among the BPDs 111 a to 111 d is determined in accordance with theexposure time: a:b:c:d=Ta:Tb:Tc:Td.

In the BPD 111 a, BPD 111 b, and BPD 111 c, the gate voltages of thetransfer transistors 112 are increased after the resetting so as to turnon the overflow path OFP.

In a period from when the BPD 111 a is reset to when the BPD 111 c isreset, a negative potential (e.g., −1 V) is applied to the transfer gateof the BPD 111 a.

In a period from when the BPD 111 b is reset to when the BPD 111 c isreset, a negative potential is applied to the transfer gate of the BPD111 b.

In this way, the period in which electric charge is generated from thetransistor interface is reduced, and introduction of the electric chargeinto the BPDs 111 is suppressed.

Since the overflow path is closed in the periods in which the negativevoltage is applied to the transfer gates of the BPD 111 a and the BPD111 b, the overflowing electric charge flows into the adjacent BPD 111 cand BPD 111 d.

Note that the periods in which the negative potential is applied to thetransfer gates of the BPD 111 a and the BPD 111 b are prior to theresetting of the BPD 111 c and the BPD 111 d.

If the overflowing electric charge flows into the BPD 111 c and the BPD111 d, all of the overflowing electric charge is discharged to the powersupply by the resetting, and the electric charge is not introduced intoa signal to be obtained.

At the same time, a positive potential or a ground potential (0 V) isapplied to the transfer gates of the BPD 111 c and the BPD 111 d inperiods prior to the resetting of the BPD 111 c and the BPD 111 d. Thus,the overflow path is opened.

Accordingly, even when the BPD 111 c and the BPD 111 d are saturated,the overflowing electric charge is discharged through the floatingdiffusion FD to the power supply.

As described above, according to the arrangement and a driving method ofthe pixel circuit of the fourth embodiment, in addition to theadvantages of the third embodiment, the amount of electric chargegenerated at the transistor interface and introduced into a BPD with ahigh sensitivity is reduced, thereby obtaining a correct output value.

5. Fifth Embodiment

Next, a fifth embodiment of the present invention will be described.

The overall structure of a CMOS image sensor according to the fifthembodiment can be the structure illustrated in FIG. 1, as in the firstembodiment.

The structure of a pixel circuit according to the fifth embodiment canbe the structure illustrated in FIG. 2, as in the first embodiment.

The arrangement of the pixel circuit according to the fifth embodimentcan be the arrangement illustrated in FIG. 3, as in the firstembodiment.

The sensitivities a to d of the BPDs 111 a to 111 d according to thefifth embodiment are different, as in the first embodiment.

The output signal and the dynamic range of a pixel according to thefifth embodiment are the same as the first embodiment, as illustrated inFIG. 4.

In the fifth embodiment, overflowing electric charge generated in theBPD 111 is absorbed by a vertical overflow drain (VOD).

FIGS. 21A and 21B are diagrams describing an overflow path of the fifthembodiment.

FIG. 21A is a top view of a pixel according to the fifth embodiment, andFIG. 21B is a sectional view of the BPD 111, the transfer transistor(TG) 112, and the floating diffusion FD taken along line XXIB-XXIBillustrated in FIG. 21A.

Also, FIG. 22 includes diagrams describing the overflow path of thefifth embodiment and illustrating the potential of electrons along lineXXIB-XXIB illustrated in FIG. 21A.

As illustrated in FIGS. 21B and 22, in the fifth embodiment, theoverflowing electric charge OFC is discharged to an N substrate by usinga P-well and the N substrate as the overflow path OFP.

Specifically, the potential of a substrate voltage VSUB is set so that,in the P-well surrounding the BPD 111, a portion that separates N+ ofthe BPD 111 and the N substrate becomes the lowest.

In this way, if the electric charge accumulated in the BPD 111 exceeds acertain amount, the exceeding amount is discharged through the VOD tothe N substrate.

In contrast, when the BPD 111 is used under a condition that the BPD 111is not saturated, it is unnecessary to discharge the overflowingelectric charge from the overflow path.

In such a case, the potential of the substrate voltage VSUB is set sothat the potential of P-well between the BPD 111 and the N substratebecomes higher by reducing a voltage applied to the N substrate. In thisway, the number of saturated electrons in the BPD 111 can be increased.

As described above, according to the fifth embodiment, the followingadvantages can be achieved in the CMOS image sensor whose dynamic rangeis extended by adding outputs from multiple BPDs with differentsensitivities.

According to the fifth embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD to thepower supply.

Also, by changing the potential of P-well that separates N+ of a BPD andthe N substrate depending on whether the BPD is saturated or not, thenumber of saturated electrons in the BPD can be increased when the BPDis not saturated.

6. Sixth Embodiment

Next, a sixth embodiment of the present invention will be described.

The overall structure of a CMOS image sensor according to the sixthembodiment can be the structure illustrated in FIG. 1, as in the firstembodiment.

The structure of a pixel circuit according to the sixth embodiment canbe the structure illustrated in FIG. 2, as in the first embodiment.

FIG. 23 is a diagram illustrating an arrangement example of a pixelcircuit according to the sixth embodiment.

In the example in FIG. 23, the BPDs 111 a to 111 d are arranged in asquare of 2×2 in each pixel. The floating diffusion FD is arranged inthe center of the BPDs 111 a to 111 d. The column reading circuit 130includes an ADC provided in each column.

The BPDs 111 a to 111 d have different sensitivities a to d. Thesensitivities of the BPDs 111 a to 111 d can be changed, for example, byproviding ND filters and changing the amount of incident light, or bychanging the exposure time.

Signals detected by the BPDs 111 a to 111 d are added by an ADC in eachcolumn, and the sum signal is output. A horizontal overflow drain (HOD)that discharges overflowing electric charge is connected to each BPD111. The HOD is shared by the adjacent BPDs 111.

In the sixth embodiment, overflowing electric charge generated in eachBPD 111 is discharged by using the horizontal overflow drain (HOD) as anoverflow path.

FIGS. 24A and 24B are diagrams describing an overflow path of the sixthembodiment.

FIG. 24A is a top view of a pixel according to the sixth embodiment, andFIG. 24B is a sectional view of the BPD 111, the transfer transistor(TG) 112, and the floating diffusion FD taken along line XXIVB-XXIVBillustrated in FIG. 24A.

Also, FIG. 25 is a diagram describing the overflow path of the sixthembodiment and illustrating the potential of electrons along lineXXIVB-XXIVB illustrated in FIG. 24A.

A method of discharging overflowing electric charge in a pixel accordingto the sixth embodiment will be described with reference to FIG. 25.

In P-well surrounding the BPD 111, the potential of a portion thatseparates N+ of the BPD 111 and N+ of the HOD is the lowest.

In this way, if the electric charge accumulated in the BPD 111 exceeds acertain amount, the exceeding amount is discharged through the HOD tothe N substrate.

As described above, according to the sixth embodiment, the followingadvantages can be achieved in the CMOS image sensor whose dynamic rangeis extended by adding outputs from multiple BPDs with differentsensitivities.

That is, according to the sixth embodiment, even under the conditionthat a BPD with a high sensitivity is saturated, a correct output valuecan be obtained by discharging overflowing electric charge from the BPDthrough the horizontal overflow drain (HOD) to the power supply.

7. Seventh Embodiment

Next, a seventh embodiment of the present invention will be described.

The overall structure of a CMOS image sensor according to the seventhembodiment can be the structure illustrated in FIG. 1, as in the firstembodiment.

FIG. 26 is a diagram illustrating an example of a pixel circuit of theCMOS image sensor according to the seventh embodiment.

A pixel circuit 110B according to the seventh embodiment includes, inaddition to the structure of the pixel circuit 110A of the firstembodiment, overflow transistors 116 a to 116 d (OFGa to OFGd) forprocessing overflowing electric charge generated in the BPDs 111 a to111 d.

The BPDs 111 a to 111 d are connected to the power supply line LVDD viathe overflow transistors 116 a to 116 d (OFGa to OFGd), respectively. Acertain potential Vref is applied to the gates of the overflowtransistors 116 a to 116 d (OFGa to OFGd).

FIG. 27 is a diagram illustrating an arrangement example of the pixelcircuit according to the seventh embodiment.

In the example in FIG. 27, the BPDs 111 a to 111 d are arranged in asquare of 2×2 in each pixel. The floating diffusion FD is arranged inthe center of the BPDs 111 a to 111 d. The column reading circuit 130includes an ADC provided in each column.

The BPDs 111 a to 111 d have different sensitivities a to d. Thesensitivities of the BPDs 111 a to 111 d can be changed, for example, byproviding ND filters and changing the amount of incident light, or bychanging the exposure time. Signals detected by the BPDs 111 a to 111 dare added by an ADC in each column, and the sum signal is output.

The overflow transistors 116 a to 116 d (OFGa to OFGd) are provided inaccordance with the BPDs 111 a to 111 d, respectively. The overflowtransistors 116 a to 116 d (OFGa to OFGd) each share a horizontaloverflow drain (HOD) that discharges overflowing electric charge withthe adjacent BPDs 111.

In the seventh embodiment, overflowing electric charge generated in eachBPD 111 is discharged by using the horizontal overflow drain (HOD).

FIGS. 28A and 28B are diagrams describing an overflow path of theseventh embodiment.

FIG. 28A is a top view of a pixel according to the seventh embodiment,and FIG. 28B is a sectional view of the BPD 111, the transfer transistor(TG) 112, and the floating diffusion FD taken along line XXVIIIB-XXVIIIBillustrated in FIG. 28A.

Also, FIG. 29 includes diagrams describing the overflow path of theseventh embodiment and illustrating the potential of electrons alongline XXVIIIB-XXVIIIB illustrated in FIG. 28A.

As illustrated in FIGS. 28A, 28B, and 29, in the seventh embodiment,overflowing electric charge OFC is discharged by using the overflow gate(OFG) and the horizontal overflow drain (HOD) as the overflow path OFP.

A method of discharging overflowing electric charge in a pixel accordingto the seventh embodiment will be described with reference to FIG. 29.

When a BPD 111 with a high sensitivity is saturated, the potential Vrefapplied to the gate electrode of the overflow gate (OFG) is set asfollows.

That is, the potential Vref is set so that the potential of a channel ofthe overflow transistor 116 (OFG) becomes lower than that of a channelof the transfer transistor (TG) 112 or P-well (not illustrated).

In this way, if a potential that is of a certain amount or greater isaccumulated in the BPD 111, the exceeding overflowing electric charge isdischarged through the overflow transistor 116 (OFG) to the overflowdrain (HOD).

In contrast, under a condition that none of the BPDs 111 is saturated,the potential of the transfer control line LTRG may be set so that thepotentials of channels of the gates of the overflow transistors 116(OFG) become higher.

In this way, the number of saturated electrons in the BPDs 111 can beincreased.

As described above, according to the seventh embodiment, the followingadvantages can be achieved in the CMOS image sensor whose dynamic rangeis extended by adding outputs from multiple BPDs with differentsensitivities.

According to the seventh embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD throughthe horizontal overflow drain (HOD) to the power supply.

Also, by changing the potential of a channel of the overflow gate of aBPD depending on whether the BPD is saturated or not, the number ofsaturated electrons in the BPD can be increased when the BPD is notsaturated.

As described above, according to the first to seventh embodiments of thepresent invention, the following advantages can be achieved in the CMOSimage sensors whose dynamic ranges are extended by adding outputs frommultiple BPDs with different sensitivities.

According to the first and second embodiments, under the condition thata BPD with a high sensitivity is saturated, a correct output value canbe obtained by discharging overflowing electric charge from the BPD tothe power supply.

By using the transfer transistors and the floating diffusion FD as theoverflow path, the overflowing electric charge can be appropriatelyprocessed without increasing the area.

By separating the overflow path from the transistor interface,introduction of noise due to the surface level can be prevented.

By reading signals from the BPDs in descending order of sensitivity, theoverflowing electric charge can be prevented from being introduced intothe floating diffusion FD, and a correct output value can be obtained.

According to the third embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD to thepower supply.

By using the transfer transistors and the floating diffusion FD as theoverflow path, the overflowing electric charge can be appropriatelyprocessed without reducing the size of the BPDs or the number of pixelsor without increasing the chip area.

By applying a low voltage to the gate of the transfer transistorconnected to a BPD with a low sensitivity, noise from the surface levelof the transistor interface can be prevented from being introduced intothe BPD, and a correct output value can be obtained.

Even when noise from the surface level is introduced into a BPD with ahigh sensitivity, a correct output can be obtained by performing acorrection using the output value read from a BPD with a lowsensitivity.

According to the arrangement and the driving method of the pixel circuitof the fourth embodiment, in addition to the advantages of the thirdembodiment, the amount of electric charge generated at the transistorinterface and introduced into a BPD with a high sensitivity is reduced,thereby obtaining a correct output value.

According to the fifth embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD to thepower supply.

Also, by changing the potential of P-well that separates N+ of a BPD andthe N substrate depending on whether the BPD is saturated or not, thenumber of saturated electrons in the BPD can be increased when the BPDis not saturated.

According to the sixth embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD throughthe horizontal overflow drain (HOD) to the power supply.

According to the seventh embodiment, even under the condition that a BPDwith a high sensitivity is saturated, a correct output value can beobtained by discharging overflowing electric charge from the BPD throughthe horizontal overflow drain (HOD) to the power supply.

Also, by changing the potential of a channel of the overflow gate of aBPD depending on whether the BPD is saturated or not, the number ofsaturated electrons in the BPD can be increased when the BPD is notsaturated.

The CMOS image sensors according to the embodiments are not particularlylimited. For example, the CMOS image sensors can be configured as CMOSimage sensors including, for example, column-parallel ADCs.

8. Eighth Embodiment

FIG. 30 is a block diagram illustrating a structure example of asolid-state image pickup device (CMOS image sensor) includingcolumn-parallel ADCs according to an eighth embodiment.

As illustrated in FIG. 30, a solid-state image pickup device 300includes a pixel array section 310 serving as an image pickup unit, arow selecting circuit 320 serving as a pixel driving unit, a horizontaltransfer scanning circuit 330, and a timing control circuit 340.

The solid-state image pickup device 300 further includes an ADC group350, a digital-to-analog converter (hereinafter abbreviated as “DAC”)360, an amplifier circuit (S/A) 370, and a signal processing circuit380.

The pixel array section 310 is configured by arranging pixels, such asthose illustrated in FIG. 2, each including a photodiode and anintra-pixel amplifier, in a matrix.

Also, in the solid-state image pickup device 300, the following circuitsare arranged as control circuits for sequentially reading signals fromthe pixel array section 310.

That is, in the solid-state image pickup device 300, the timing controlcircuit 340 which generates an internal clock, the row selecting circuit320 which controls row addresses and row scanning, and the horizontaltransfer scanning circuit 330 which controls column addresses and columnscanning are arranged as control circuits.

The ADC group 350 includes column-parallel ADCs, each including acomparator 351, a counter 352, and a latch 353.

The comparator 351 compares a reference voltage Vslop that is a rampwaveform (RAMP), which is obtained by changing a reference voltagegenerated by the DAC 360 to be a stepped voltage, with an analog signalobtained for each row line from pixels through column signal lines.

The counter 352 counts a comparison time of the comparator 351.

The ADC group 350 has an n-bit digital signal converting function andincludes column-parallel ADC blocks arranged in the individual verticalsignal lines (column lines).

An output of each latch 353 is connected to a horizontal transfer line390 with, for example, an 2n-bit width.

Also, 2n amplifier circuits (S/A) 370 and signal processing circuits380, the number of which corresponds to the horizontal transfer line390, are arranged.

In the ADC group 350, an analog signal read to a vertical signal line(potential Vsl) is compared with a reference voltage Vslop (slopewaveform that changes linearly at a certain gradient) by using thecomparator 351 arranged in each column.

On this occasion, the counter 352 arranged in each column, as in thecomparator 351, is operating. Since the potential Vslop with a rampwaveform and the counter value change with a one-to-one correspondence,the potential of the vertical signal line (analog signal) Vsl isconverted into a digital signal.

A change of the reference voltage Vslop corresponds to conversion of achange in voltage into a change in time. That time is counted using acertain cycle (clock), thereby obtaining a digital signal.

When the analog electric signal Vsl and the reference voltage Vslopintersect, the output of the comparator 351 is inverted, and the inputclock of the counter 352 is stopped. Accordingly, AD conversion iscompleted.

After the foregoing AD converting period is completed, the horizontaltransfer scanning circuit 330 inputs data held in the latch 353 to thesignal processing circuit 380 via the horizontal transfer line 390 andthe amplifier circuit (S/A) 370, thereby generating a two-dimensionalimage.

In this manner, column-parallel output processing is performed.

The solid-state image pickup device with the foregoing advantages can beapplied as an image pickup device of a digital camera or a video camera.

9. Ninth Embodiment

FIG. 31 is a diagram illustrating an example of the configuration of acamera system to which a solid-state image pickup device according to anembodiment of the present invention is applied.

A camera system 400 includes, as illustrated in FIG. 31, an image pickupdevice 410 to which the CMOS image sensor (solid-state image pickupdevice) 100 or 300 according to an embodiment of the present inventionis applicable.

The camera system 400 further includes an optical system that directsincident light to a pixel region of the image pickup device 410 (thatforms an image of a photographic subject), such as a lens 420 that formsan image from the incident light (image light) on an image pickup face.

The camera system 400 also includes a drive circuit (DRV) 430 thatdrives the image pickup device 410, and a signal processing circuit(PRC) 440 that processes an output signal of the image pickup device410.

The drive circuit 430 includes a timing generator (not illustrated inthe drawings) that generates various timing signals including a startpulse that drives circuits in the image pickup device 410, and a clockpulse. The drive circuit 430 drives the image pickup device 410 using acertain timing signal.

Also, the signal processing circuit 440 applies certain signalprocessing to an output signal of the image pickup device 410.

An image signal processed in the signal processing circuit 440 isrecorded on a recording medium, such as a memory. A hard copy of theimage information recorded on the recording medium is generated using aprinter or the like. Also, the image signal processed in the signalprocessing circuit 440 is displayed as a moving image on a monitorincluding a liquid crystal display or the like.

As described above, in an image pickup apparatus such as a digital stillcamera, a low-power consumption and highly precise camera can berealized by including the above-described image pickup device 100 or 300as the image pickup device 410.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2009-027895 filedin the Japan Patent Office on Feb. 9, 2009, the entire content of whichis hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A solid-state image pickup device comprising: apixel circuit having a plurality of unit pixels, each unit pixelincluding a photoelectric conversion element, a transfer element, and areset element, the pixel circuit further including a node element; andan overflow path provided between the photoelectric conversion elementand the node element, wherein the overflow path is configured to absorboverflowing electric charge from the photoelectric conversion elementthrough the transfer element during an on-state of the reset elementoccurring during an exposure period of the photoelectric conversionelement, and wherein the pixel circuit is configured to read signalsfrom a plurality of photoelectric conversion elements of the pluralityof unit pixels in a descending order of sensitivity of the plurality ofphotoelectric conversion elements.
 2. The solid-state image pickupdevice according to claim 1, wherein the photoelectric conversionelement is configured to generate an electric charge in response toincident light.
 3. The solid-state image pickup device according toclaim 1, wherein the transfer element is configured to transfer anelectric charge to the node element.
 4. The solid-state image pickupdevice according to claim 1, wherein the node element is configured toamplify an electric charge transferred from the photoelectric conversionelement and to output an image signal.
 5. The solid-state image pickupdevice according to claim 1, wherein, in the exposure period, a powersupply voltage is supplied to the node element, and the overflowingelectric charge which has been discharged to the node element flows intoa power supply.
 6. The solid-state image pickup device according toclaim 1, wherein a potential between the photoelectric conversionelement and a transistor interface corresponding to the transfer elementis highest within the overflow path.
 7. The solid-state image pickupdevice according to claim 1, wherein the pixel circuit is configured tocontrol turning on/off of the overflow path by controlling a gatevoltage of the transfer element.
 8. The solid-state image pickup deviceaccording to claim 7, wherein the overflow path is turned on only in thephotoelectric conversion element where the overflowing electric chargeis generated.
 9. The solid-state image pickup device according to claim1, wherein the overflow path is configured to discharge the overflowingelectric charge from the plurality of photoelectric conversion elementsto the node element.
 10. The solid-state image pickup device accordingto claim 1, wherein the node element is a floating diffusion.
 11. Thesolid-state image pickup device according to claim 1, wherein theoverflow path is configured to discharge the overflowing electric chargegenerated in the photoelectric conversion element by using an overflowdrain as the overflow path.
 12. The solid-state image pickup deviceaccording to claim 1, wherein, the overflow path is configured todischarge the overflowing electric charge generated in the photoelectricconversion element by using an overflow transistor and an overflow drainas the overflow path.
 13. The solid-state image pickup device accordingto claim 1, wherein a first depth, from a substrate front face, at whichthe overflow path is formed in the transfer element is deeper than asecond depth, from the substrate front face, at which a transistorinterface is formed.
 14. A camera system comprising: a solid-state imagepickup device; and an optical system configured to form an image of aphotographic subject on the solid-state image pickup device, wherein thesolid-state image pickup device includes: a pixel circuit having aplurality of unit pixels, each unit pixel including a photoelectricconversion element, a transfer element, and a reset element, the pixelcircuit further including a node element, and an overflow path providedbetween the photoelectric conversion element and the node element,wherein the overflow path is configured to absorb overflowing electriccharge from the photoelectric conversion element through the transferelement during an on-state of the reset element occurring during anexposure period of the photoelectric conversion element, and wherein thepixel circuit is configured to read signals from a plurality ofphotoelectric conversion elements of the plurality of unit pixels in adescending order of sensitivity of the plurality of photoelectricconversion elements.
 15. The camera according to claim 14, wherein thephotoelectric conversion element is configured to generate an electriccharge in response to incident light.
 16. The camera according to claim14, wherein the transfer element is configured to transfer an electriccharge to the node element.
 17. The camera according to claim 14,wherein the node element is configured to amplify an electric chargetransferred from the photoelectric conversion element and to output animage signal.
 18. The camera according to claim 14, wherein, in theexposure period, a power supply voltage is supplied to the node element,and the overflowing electric charge which has been discharged to thenode element flows into a power supply.
 19. The camera according toclaim 14, wherein a potential between the photoelectric conversionelement and a transistor interface corresponding to the transfer elementis highest within the overflow path.
 20. The camera according to claim14, wherein the pixel circuit is configured to control turning on/off ofthe overflow path by controlling a gate voltage of the transfer element.21. The camera according to claim 20, wherein the overflow path isturned on only in the photoelectric conversion element where theoverflowing electric charge is generated.
 22. The camera according toclaim 14, wherein the overflow path is configured to dischargeoverflowing electric charge from a plurality of photoelectric conversionelements to the node element.
 23. The camera according to claim 14,wherein the node element is a floating diffusion.
 24. The cameraaccording to claim 14, wherein the overflow path is configured todischarge the overflowing electric charge generated in the photoelectricconversion element by using an overflow drain as the overflow path. 25.The camera according to claim 14, wherein, the overflow path isconfigured to discharge the overflowing electric charge generated in thephotoelectric conversion element by using an overflow transistor and anoverflow drain as the overflow path.
 26. The camera according to claim14, wherein a first depth, from a substrate front face, at which theoverflow path is formed in the transfer element is deeper than a seconddepth, from the substrate front face, at which a transistor interface isformed.